Dynamic composition of an execution environment from multiple immutable file system images

Abstract

A virtual file system is formed configured to enable the dynamic composition of immutable file system images. A file system containing a software distribution is divided into a plurality of mutually exclusive sub-trees. Each sub-tree includes a portion of the software distribution. An immutable file system image is formed for each sub-tree. During the booting of an operating system, a virtualization engine intercedes in the boot process to mount the immutable file system images to independent directories of the root file system. Upon request the virtualization engine, during run-time, combines virtual entries corresponding to immutable file system images so as to resemble the original software distribution.

1. A method for the execution of immutable file system images on a computer system, the method comprising:
 establishing a root file system; 
 forming a plurality of immutable file system images wherein each of the plurality of immutable file system images is associated with a mutually exclusive sub-tree of a file system tree; 
 attaching each of a plurality of immutable file system images to an independent directory of the root file system prior to attaching other file systems to the root file system; 
 generating a virtualization data structure corresponding to an empty file system tree; 
 creating at run-time, in the virtualization data structure, a virtual directory including a virtual entry for each of the plurality of immutable file system images wherein each virtual entry includes file system structure of the mutually exclusive sub-tree; and 
 joining functionally at run-time two or more of the plurality of immutable file system images by combining the virtual entries for each of the two or more of the plurality of immutable file system images. 
 
   
   
     2. The method of  claim 1  wherein root file system establishment is halted prior to mounting of any existing file systems. 
   
   
     3. The method of  claim 1  further comprising examining each of the plurality of immutable file system images to identify a mutual directory common to two or more of the plurality of immutable file system images wherein the mutual directory includes a separate entry from each of the two or more of the plurality of immutable file system images. 
   
   
     4. The method of  claim 1  wherein authenticity of each of the plurality of immutable file system images can be verified prior to attaching. 
   
   
     5. The method of  claim 1  further comprising writing to a memory an actual location identifier for each virtual entry of the virtual directory. 
   
   
     6. The method of  claim 5  wherein the actual location identifier for each virtual entry of the virtual directory can be replaced while the computer system is running. 
   
   
     7. The method of  claim 6  wherein the virtualization data structure is accessible via a virtual file system (“VFS”) application programming interface (“API”) and wherein the VFS API determines entry locations using the actual location identifier. 
   
   
     8. The method of  claim 5  further comprising regenerating the virtualization data structure when a valid attempt to modify at least one of the separate entries in the virtual directory is received. 
   
   
     9. The method of  claim 1  wherein the file system tree includes a software distribution. 
   
   
     10. The method of  claim 9  wherein combining the virtual entries of the two or more of the plurality of immutable file system images functionally recreates the software distribution. 
   
   
     11. A system for providing a virtual file system tree for dynamic composition of a plurality of read-only file system images containing software in pre-installed form operable in a machine capable of executing instructions embodied as software, the system comprising:
 a hardware processor running an operating system having an initial root file system tree in memory accessible by the hardware processor; and 
 a plurality of software portions selectively executable by the hardware processor, wherein 
 one of said software portions is configured to cause the system to mount each of the plurality of read-only file system images under an independent directory of the initial root file system; 
 one of said software portions is configured to cause the system to create a virtual file system tree separate from the initial root file system tree; 
 one of said software portions is configured to cause the system to form as part of the virtual file system tree a mutual directory wherein the mutual directory includes a virtual file having a path to each of the plurality of read-only file system images; and 
 one of said software portions configured to cause the system to replace at least one of the virtual files while the operating system is running. 
 
   
   
     12. The system of  claim 11 , further comprising responsive to replacing the initial root file system with the virtual file system tree, mounting additional read-write file system trees to the virtual file system tree. 
   
   
     13. The system of  claim 11  wherein one of said software portions is configured to form a read-only file system image by combining a plurality of virtual files.